Method and apparatus for inspecting cavities



March 3, 1953 E. D. M ARTHUR METHOD AND APPARATUS FOR INSPECTING CAVITIES Filed .July 1, 1948 Fig.2.

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Patented Mar. 3, 1953 UNITED STATES PATENT OFFICE METHOD AND APPARATUS FOR INSPECTING CAVITIES New York Application July 1, 1948, Serial No. 36,352

This invention relates to a method and apparatus for inspecting cavities having electrically conducting walls for dimensional defects, such as cavities in metal castings and the like,

A principal object of the invention is to provide a new and improved method and apparatus for inspecting cavities in metal castings, and other cavities in electrically conducting materials, for dimensional defects. Such cavities may have various shapes with relatively small exterior openings, so that the introduction of calipers and other conventional measuring devices is often difficult or impossible. Other objects and advantages of this invention will become apparent as the description proceeds.

This invention utilizes microwave electro-magnetic energy as a means for inspecting cavities. The term microwave electromagnetic energy as used herein refers to electromagnetic waves havlllg a frequency of 3 10 to 3 X 10 cycles per secend, which corresponds to a wavelength of approximately 100 cm. to 1 cm. The precise band- Width of frequencies to be employed depends upon the size and shape of the cavity to be inspected, as hereinafter explained.

It is known that cavities having electrically conducting walls can be excited with electromagnetic energy; and that at certain frequencies cavities so excited exhibit the phenomena of resonance, in a manner similar to tuned circuits. In general, each cavity has .a number of frequencies at which it is resonant depending upon its shape and dimensions. If the shape or dimensions of the cavity are changed, one or more of these resonant frequencies are, in general, also changed. Thus, the dimensional similarity of two cavities can be checked by comparing the frequencies at which each is resonant. It has also been observed:

that the resonance effects .are more pronounced at some of the resonant frequencies of any particular cavity than at other resonant frequencies. This provides an additional means of. comparing two cavities.

This method of inspection is particularly valuable where a large number of identical castings must bemade, as in a manufacturing process. .A casting having a cavity which is known to be dimensionally perfect can be used as a stand ard. Other castings can then be quickly inspected in the manner hereinafter described by comparing their resonant frequencies with the resonant frequencies of the standard casting.

The features of this invention which are believed to be novel and patentable are pointed out in the claims forming a part of this specification. For a 2 Claims. (Cl. 175-183) better understanding of the invention, reference is made in the following description to the :accompanying drawing in which Fig. 1 is a schematic illustration of a simple apparatus for carrying out this method of inspection; Fig. 2 is a schematic illustration of another form of such apparatus adapted to indicate the resonant frequencies panoramically for more rapid inspection; and Fig. 3 is a form of such apparatus adapted for direct comparison of two cavities.

Referring now to Fig. 1 of the drawing, a metallic casting i, illustrated in cross section, contains a cavity which is to be inspected. A variable frequency oscillator 2 provides microwave electromagnetic energy. Oscillator 2 has a frequency range suflicient to cover the more important resonant frequencies of the cavity to be tested. It has been found that the lowest resonant frequency of a cavity is usually a frequency having a wave length of one to two times the width of the cavity, depending upon its shape. For example, the lowest resonant frequency of a hollow spherical cavity is 1.14 times its diameter; that of a cylindrical cavity is 1.31 times its diameter; and that of a square cavity is 1.42 times its width, A frequency range starting with this lowest resonant frequency and extending to two or three times this frequency will usually be desirable in making the inspection. Frequency adjustment dial 3 schematically represents means by which the oscillator frequency can be varied. Electromagnetic energy from the oscillator is transmitted to the cavity by a coaxial cable 4 or by a wave guide. An antenna 5 radiates the enery into the cavity while a shield 6 prevents loss of energy by radiation in other directions. A fre quency meter 1 is provided to accurately measure the frequency of energy transmitted to the cavity. If the variable frequency oscillator is very accurately calibrated as to frequency, frequency meter 1 can be omitted.

Means to meter the relative magnitude of energy transmitted to the cavity is provided in the form of a milliammeter 3 connected in series with the plate voltage supply 9 of the oscillator. As the power drawn from the oscillator increases, its plate current will change. This is indicated by milliammeter 8. The points of maximum change of plate current correspond to resonant frequencies of the cavity, and the relative magnitudes of plate current at these points indicate the relative degree of resonant effects present at the corresponding frequencies.

If two cavities are inspected with this apparatus, and the resonant frequencies and relative degrees of resonance effects of each arerecorded, the results of the two tests can be compared and it will be readily apparent whether or not the two cavities are dimensionally identical. In this manner, any number of cavities can be compared with a standard for inspection purposes.

In Fig. 2 a form of apparatus is shown for making these inspections much more rapidly. The electrically conducting material II) here illustrated has a cavity which is filled with a non-conducting material II. Such conditions are encountered in industry where cavities are often filled with plastic or other non-conducting material. Electromagnetic energy is supplied by an oscillator I2, transmitted by a coaxial cable I3 and radiated into the cavity by antenna I4. Shield I5 prevents loss of energy radiated in other directions.

To eliminate the manual frequency adjustment required to operate the apparatus of Fig, 1, oscillator I2, Fig. 2, is frequency-modulated by a modulating voltage supplied from modulator I6. As the modulating voltage varies, the frequency of electromagnetic energy supplied by oscillator I2 likewise varies, so that the electromagnetic energy supplied by oscillator I2 is swept across the desired frequency range durin each cycle of the modulating voltage. Modulator it provides a modulating voltage which is much lower in frequency than the microwave energy supplied by oscillator 52. For example, the modulating voltage can have a frequency of 60 cycles per second in which case modulator i5 may be an electrical outlet connected to a commercial power line. Preferably, however, modulator It provides a modulating voltage having sawtooth waveform, so that the output of oscillator I2 is swept relatively slowly across the desired frequency range in one direction and then quickly returned to the original frequency.

To meter the microwave energy transmitted to the cavity, a R.-F. metering element I! is inserted in cable I3. This metering element may, for example, comprise a small crystal rectifier connected to rectify a portion of the R.F. current flowing through cable I3, or may be one of other R.-F. detecting elements known in the art. This detecting element provides a metering voltage which is dependent in value upon the energy transfer to cavity I I.

A cathode ray oscillograph tube I8 is provided to panoramically indicate the resonant frequencies of the cavity. Connections I9 apply the modulating voltage to the horizontal deflection plates of the oscillograph tube. The metering voltage, provided by metering element I1, is amplified by amplifier 20 and applied to the vertical deflecting plates of oscillator tube-IBM With this arrangement, the instantaneous hori.,

zontal position of the indicatingspot on the face of the oscillograph tube depends upon the instantaneous value of the modulating voltage, and hence upon th instantaneous frequency of the microwave energy generated by oscillator I2. The instantaneous vertical position of the indicating standard cavity. If the standard trace has been marked on theface of the tube, any substantial 1 differences in the two traces are at once apparent,

so that a large number of cavities can be quickly and accurately inspected.

In Fig. 3, a form of apparatus is shown for directly comparing two cavities. One of these cavities may be the standard sample, and the 'two cavities by antennae 21 and 28.

other a cavity to be tested. This form of apparatus is especially desirable where cavities of several different sizes or shapes are to be tested with the same apparatus, so that it is not desirable to permanently mark any one trace upon the face of the oscillograph tube. The apparatus of Fig. 3 is also more accurate, since both cavities are tested at the same time and hence any tendency of errors to arise because of calibration drift, or other factors associated with the apparatus tending to cause a change in the trace, are eliminated.

Referring now to Fig. 3, metal castings 22 and 23 have respective cavities which are to be compared. Microwave electromagnetic energy is supplied by frequency modulated oscillator 24, transmitted by cables 25 and 26, and radiated into the Shields 29 and 30 prevent loss of electromagnetic energy radiated in undesired directions. Modulator 3I provides a modulating voltage which frequency modulates the electromagnetic energy as in the apparatus of Fig. 2. Two R.-F. metering elements 32 and 33 are respectively positioned in cables 25 and 26, and provide metering voltages dependent in value upon the respective magnitudes of energy transmitted to the two cavities.

spot depends upon the instantaneous value of the An electronic switch 34 alternately applies the metering voltages to the input of amplifier 35, which amplifies the composite metering voltage and applies it to the vertical deflection plates of cathode ray oscillograph tube 35. Connections 3'! apply the modulating voltage to the horizontal deflection plates of tube 36.

Preferably, electronic switch 34 is synchronized with the modulator by connections as shown, so

that the metering voltage from element 32 is applied to the vertical deflecting plates 36 during every second horizontal sweep of the indicating spot across the face of the oscillograph tube, and the metering voltage from element 33 is applied to the vertical deflecting plates during the other horizontal sweeps of the spot. Two traces 3'! and 38 thus appear on the face of tube 36. These two traces respectively portray the resonance characteristics of the two cavities. To the eye, both traces appear to be on the face of the tube continuously, due to persistence of vision of the eye and to persistence of the phosphor coating of the tube face, and so can be readily compared. The same result can be obtained if switch 34 operates out of synchronism with modulator 3!, but at a rate so rapid that persistence of vision and of the phosphor causes both patterns to appear continuously upon the tube face. Electronic switches capable of operating in either of the above ways are commercially available.

Having described the principle of this invention and the best manner in which I have contemplated applying that principle, I wish it to be understood that the apparatus described is illustrative only, and that other means can be employed without departing from the true scope of the invention defined by the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of inspecting cavities having electrically conductive walls for dimensional defects, comprising exciting each cavity to be inspected with microwave electromagnetic energy, varying the frequency of such energy through a plurality of resonances of such cavity, and metering the relative magnitude of energy transferred to the cavity at different frequencies to determine the frequencies at which such cavity is resonant and the relative magnitudes of the energy transferred to the cavity at the diiferent resonant frequencies.

2. Apparatus for inspecting cavities, comprising a source of microwave electromagnetic energy which is frequency modulated responsive to a modulating voltage of lower frequency, such modulation providing a range of frequencies which comprises a plurality of resonances of the cavity inspected, means to provide such modulating voltage, means to transmit such microwave electromagnetic energy to at least one cavity to be inspected, means to provide a metering voltage dependent in value upon the magnitude of energy transferred to such cavity, an oscillograph with horizontal deflection and vertical deflection voltage inputs, and connections for applying said modulating voltage to one of such inputs and said metering voltage to the other of such inputs.

ELMER D. MCARTHUR.

REFERENCES CITED The following references are of record in the file of this patent:

Physical Review, vol. 70, numbers 3 and 4, Aug. 1 and 15, 1946, pages 213-218.

Technique of Microwave Measurements by Montgomery, copyright 1947, by McGraw-Hill Book Co., pages 403-407.

Resonant-Cavity Measurements by Sproull and Linder, RCA Laboratories R 209, reprinted from Proceedings of the I. R. E., May 1946.

Microwave Measurements and Test Equipments" by Gaffney, Proceedings of the I. R. and Waves and Electrons. October 1946, p 786. 

